See http://www.wcha.org/forums/index.php?threads/12797/ for a previous request with this information. Feel free to reply here or there if you have any other questions. (I have two 18 foot Otcas in the garage at home but will have to wait until tonight before I can get the thwart specifications for you.)
The original scan is in GIF format so the thumbnail image doesn't display correctly in the Internet Explorer but you should still be able to click on it to see the full size image. This works for me so please reply here if you are having another problem. It is also included below in JPG format if that helps.